1. Field of the Invention
This invention relates to an improved method and apparatus for regenerating a cracking catalyst. More particularly, this invention relates to an improved fluid catalytic regenerator design and the method of operation thereof.
2. Discussion of the Prior Art
In recent years the design and operation of fluid cracking operations with an adjacent catalyst regeneration system has gone through some unusual design transitions with a view to improving the efficiency of the combination operation as well as the product distribution obtained from such an operation. In particular, the designs have been concerned with utilizing fluid crystalline aluminosilicate cracking catalysts in volume to oil feed ratios which minimize the catalyst inventory of the operation, improve product selectivity, and improve the recovery of available heat generated in the catalyst regeneration system. Catalyst regeneration has been improved by increasing the catalyst bed regeneration temperature by the recycle of hot regenerated catalyst thereto and by particularly promoting the combustion of CO to CO.sub.2 therein by thermal and catalytic effects. That is, some recent design changes incorporate recycling by external pipe means of hot regenerated catalyst for admixture with cooler spent catalyst recovered from the hydrocarbon conversion operation such that the combined temperature of the mixed catalyst streams is sufficiently high to rapidly initiate coke burning and accomplish catalytic CO (carbon monoxide) burning in a substantial portion of a dense fluid bed of catalyst being regenerated. It has been found in some regeneration operations that the CO concentration in the flue gas exceeds emission standards and unburned residual carbon on regenerated catalyst becomes undesirably high; that is, above about 0.05%. Several design parameters and apparatus arrangements have been proposed to solve this problem. However, these designs often suffer from a number of problems such as high catalyst inventory, low temperature, incompletely regenerated catalyst, a lack of operating flexibility to control catalyst recycle, or employ external apparatus configurations or arrangements in an effort to effect more suitable control in the operation, thereby contributing to costs.
Some regenerator vessel designs and arrangements have been substantially increased in height, thereby increasing construction costs. In these arrangements, the circulating catalyst inventory and necessary catalyst bed hold-ups have increased and high temperature metallurgy requirements have increased. These factors contribute to increased material, maintenance and operating costs of the units. Additional operating details of these and other prior art FCC units can be found in: U.S. Pat. Nos. 2,383,636 (Wurth); 2,689,210 (Leffer); 3,338,821 (Moyer et al); 3,812,029 (Synder, Jr.); 4,093,537 (Gross et al); 4,118,338 (Gross et al); and 4,218,306 (Gross et al) as well as in Venuto et al, Fluid Catalytic Cracking with Zeolite Catalysts, Marcel Dekker, Inc. (1979). The entire contents of all of the above patents and publications are incorporated herein by reference.
In some recent designs, flexibility of FCC regenerators is improved by providing a means of recycling at least a portion of the regenerated hot catalyst into a bed of spent catalyst (Gross et al, U.S. Pat. No. 4,118,338). The recycle is accomplished by providing two concentric fluidized catalyst beds in the regenerator. The inner bed contains spent catalyst and the outer bed regenerated catalyst. The amount of the regenerated catalyst recycled into the inner bed is controlled by the pressure differential between the upflowing inner bed and the downflowing outer catalyst bed. This improved regenerator design increases flexibility of the FCC installation, allows for high catalyst recirculation ratios and substantially decreases the total inventory of the catalyst necessary for carrying out the process. However, this design, under certain operating conditions, also makes it possible for the respective catalyst beds to reverse their intended direction of flow. In addition, under some operating conditions, the regenerated catalyst may have more residual coke left thereon than might be desired.